As a type of high-energy density secondary battery, there has been proposed a non-aqueous electrolyte-type lithium secondary battery (e.g., Patent documents 1-4 listed below). The battery utilizes a phenomenon that a carbon intercalation compound of lithium can be easily formed electrochemically, and when the battery is charged, lithium in the positive electrode comprising, e.g., a chalcogenide compound such as LiCoO2, is electrochemically inserted between carbon layers in the negative electrode (doping). The carbon thus-doped with lithium function as a lithium electrode to cause a discharge, whereby the lithium is liberated (dedoped) from the carbon layers to return to the positive electrode.
In order to achieve a higher energy density in such a non-aqueous electrolyte-type lithium secondary battery, it is necessary to increase the amount of lithium dedoped and doped per unit weight of the positive electrode substance and the amount of lithium doped and dedoped per unit weight of the negative electrode substance, and further necessary to incorporate increased amounts of the positive and negative electrode substances in the secondary battery. From such a viewpoint, a graphitic material having high doping and dedoping capacity per volume has been used particularly as a negative electrode material.
In recent years, a non-aqueous electrolyte-type lithium secondary battery has been expected to be used not only as a power supply for small-size portable instruments but also as a power supply for a hybrid electrical vehicle (hereinafter abbreviated as a “HEV”). Such a HEV is loaded with an internal-combustion engine in addition to the battery as motive power supplies therefor, so that the battery is not required to supply a large amount of energy but is required to supply a high power output capable of driving the vehicle or sufficiently supplementing the motive power of the vehicle. Further, in order to achieve a lower fuel consumption, it is indispensable to effectively recover a braking energy of the vehicle and is further required to exhibit a high input capacity.
On the other hand, while the expected life of a non-aqueous electrolyte secondary battery as a power supply for small-size portable instruments is several years, a power supply system for HEVs, comprising several hundreds of cells connected in series cannot be easily exchanged in the middle of the life of the vehicle but is required to exhibit a life and a reliability comparable to the life of the vehicle, i. e., of 10 or more years.
As a means for improving the output performance of a non-aqueous electrolyte secondary battery, there has been proposed to control the electrode thickness and the particle size of the active substance (Patent document 5 listed below). More specifically, by making thinner the electrode, it becomes possible to increase the reaction area and reduce the reaction potential difference in the electrode thickness direction. As a result, it becomes possible to reduce a polarization between the surfacemost layer and a layer close to the electroconductive substrate of the electrode, thereby reducing a lowering of performance at the time of a large current discharge. However, the output is not yet sufficient and a higher output is demanded. Further, accompanying the use of a thinner electrode, larger numbers of conductive substrates and separators for the positive and negative electrodes are required than usual, and this results in a lowering of energy density of the battery, for which an improvement is also desired.
As for the reliability of a negative electrode material, a graphitic material and a graphitizable carbon material having a turbostratic texture are liable to cause a repetition of expansion and constriction of crystallites at the time of doping and dedoping of lithium, so that they are poor in reliability as a negative electrode material of non-aqueous electrolyte secondary battery used for HEVs. On the other hand, non-graphitizable carbon material causes little expansion and constriction at the time of doping and dedoping of lithium to exhibit a high cycle durability so that it is expected to be promising as a negative electrode material of non-aqueous electrolyte-type lithium secondary battery used for HEVs. However, the texture of non-graphitizable carbon is variously changed depending on the texture of a carbon precursor and heat-treatment conditions thereafter, an appropriate texture control is important for achieving good charge-discharge performances. Non-graphitizable carbon particle exhibiting good charge-discharge performances have been obtained through pulverization of a carbon precursor itself or after calcination thereof, so that it requires a lot of pulverization energy for providing a smaller particle size which is indispensable for achieving a thin layer of electrode active substance and the smaller particle size is accompanied with an increased amount of fine powder to result in a lowering in reliability of the battery. There also arises a problem that the enhancement of pulverization and removal of fine particles for providing the smaller particle size results in a remarkable lowering in pulverization efficiency.
It has been proposed to use a non-graphitizable carbon having a spherical shape as a negative electrode active substance for providing a non-aqueous electrolyte exhibiting a high energy density and less liability of short circuit due to formation of dendrite, thus exhibiting a high reliability (Patent document 6 listed below). It is intended to form a negative electrode having a uniform distribution of active substance through coating, etc., by using spherical carbon as the negative electrode active substance, and thereby to provide a negative electrode with less liability of internal short circuit due to dendrite formation and with an electrical capacity closer to a theoretical one. However, substantially no process for producing the spherical non-graphitizable carbon is disclosed. Further, the discharge capacity thereof was at most 320 mAh/g, which does not exceed the theoretical capacity of graphitic material and is not sufficiently large.
On the other hand, while it is easily conceived of carbonizing a spherical synthetic resin in order to obtain a spherical carbon material, this is actually not easy. Synthetic reins include: thermosetting resins causing polycondensation under heating and vinyl resins obtained through radical polymerization. A thermosetting resin generally provides a relatively good carbonization yield, but it forms a viscous condensate difficult to handle at an initial stage of condensation and requires further many steps for sphering thereof. A spherical non-graphitizable carbon obtained from phenolic resin as the starting material is disclosed in Patent document 7 listed below, which however does not disclose a process for producing spherical phenolic resin as the starting material. Further, the resultant spherical non-graphitizable carbon exhibited a fairly low discharge capacity of 185 mAh/g. On the other hand, vinyl resins can be obtained as spherical polymerizates through radical suspension polymerization, but most of them cause de-polymerization or thermal decomposition at the time of carbonization treatment, thus failing to leave a substantial amount of carbonization product.
Patent document 1: JP-A 57-208079
Patent document 2: JP-A 62-90863
Patent document 3: JP-A 62-122066
Patent document 4: JP-A 2-66856
Patent document 5: JP-A 11-185821
Patent document 6: JP-A 6-150927
Patent document 7: JP-A 6-20680